1. Field of the Invention
This invention relates generally to geologic survey sensors and more particularly to seismic sensors.
2. Description of the Related Art
Oil and gas exploration includes the acquisition of formation characteristics by conducting seismic surveys. When seismic surveys are conducted on land, sensors are positioned in a survey area. Well-known techniques such as using vibrator trucks or explosives are employed to generate an acoustic wave. The acoustic wave travels through earth formations and is partially reflected at formation discontinuities. Various sensor types are used to sense the reflected wave as it returns to the surface. The senor outputs a signal indicative of the wave, and a surface controller is then typically used to record the signal.
A typical sensor used is a velocity sensor, also known in the art as a geophone. A velocity sensor is a spring-mass sensor that uses relative motion between a mass and a coil to generate an analog output signal. When an acoustic wave contacts the sensor, the sensor housing moves. An internal mass suspended by a spring within the housing, tends to remain motionless as the housing moves relative to the internal mass. In a geophone, the internal mass is an electrically conductive coil having output leads and the housing contains an attached magnet. The relative motion of the magnet with respect to the coil produces a voltage output on the output leads. The resultant voltage produced is proportional to the velocity of the relative motion.
An alternative to the velocity-type geophone is an acceleration sensor called an accelerometer. Recent advances in accelerometer technology have resulted in the development of micro-electromechanical systems (xe2x80x9cMEMSxe2x80x9d) based accelerometers. These MEMS accelerometers have been used in seismic sensor modules with some performance features comparable to a geophone-based module.
A drawback of a typical MEMS sensor module is that the module is sensitive to large amplitude, short period mechanical shock known as high-g shock inputs. Such inputs are commonly encountered during handling of seismic equipment in the field during transportation and insertion (xe2x80x9cplantingxe2x80x9d) of sensor modules in the ground. These high-g shocks are typically two and one half orders of magnitude larger than seismic energy sensed by the accelerometer, which may damage or destroy accelerometers housed in the modules. High-g as used herein is distinguished from sub-g, which is defined as any input force less than 1 g (1xc3x97 the force due to gravity).
Another problem encountered in a typical accelerometer is certain noise encountered during operation caused by resonances of the module structure. There is a need for a seismic sensor having noise abatement capability for noise created by system resonance.
The present invention described below addresses some or all of the drawbacks described above by providing a seismic sensor having single or multi-axis sensitivity and which can withstand high-g shock during handling and transport, and which can subsequently reduce module noise while measuring sub-g acoustic waves when the module is planted.
In one aspect of the invention, an apparatus for sensing seismic waves in the earth is provided. The apparatus includes a housing with one or more seismic sensors disposed in the housing. At least one isolator is coupled to the one or more seismic sensors for isolating the one or more seismic sensors from high-g shock induced in the housing.
In another aspect of the invention, a seismic sensor module tolerant to high-g shock inputs is provided. The module comprises a module case and a sensor assembly housed by the module case. An inertial mass is coupled to at least one seismic sensor in the sensor assembly, and at least one isolator is coupled to the sensor assembly and the module case.
Another aspect of the invention provides a seismic sensor module that comprises a module case and a sensor assembly coupled to the module case. The sensor assembly includes at least one seismic sensor, and an inertial mass is coupled to the sensor assembly.
A sensor module tolerant to high-g shock inputs is provided in another aspect of the invention, wherein the module comprises a module case and a sensor assembly within the module case. The sensor assembly includes an inertial mass coupled to the module case, and at least one seismic sensor coupled to the inertial mass. An isolation layer is coupled to the module case and the sensor assembly such that the sensor assembly remains substantially motionless relative to the module case when an input force of less than a predetermined level is applied to the module case.
A method of isolating one or more seismic sensors in a seismic sensor assembly from high-g shock loads while maintaining sensitivity to seismic waves is provided in another aspect of the present invention. The method comprises providing a housing for the seismic sensor assembly, installing one or more seismic sensors in the housing, and providing an isolator between the one or more sensors and the housing.